Electric furnace



P. K. DEVERS ELECTRIC EURNACE April 9, 1929.

Filed July 13, 1925 fr? l/enof; Ph/Y/,b fIDel/e/"s,

atorney.

Patented Apr. 9, 1929.

UNITED STATES A 1,108,833! PATENT OFFICE.

PHILIP K. DEVEBS, OF LYNN, MASSACHUSETTS, .ASSIGNOR T GENERAL ELECTRICCOMPANY, A CORPORATION 0F NEW YORK.

ELECTRIC FURNACE.

Application med July 13, 1925. serial No. 43,199.

The present invention relates to a process and to an electric furnaceparticularly adapted for the preparation of vitreous silica,

also commonly known as fused quartz, and

5 it is an object of my invention ,to better .the product and increasethe length of life of the fusion apparatus.

As described in my prior Patent N o.

1,536,821 of May 5, 1925, fragments or particles of quartz firstarefused in a vacuum and the productl of this fusion then is fabricatedinto objects of desired form by pressing, drawing, extrusion or thelike. If material of a high quality is desired relatively high pressureis applied upon the vacuum# fused material while it is plastic toeliminate cavities occuring therein. i

The vacuum fusion heretofore was carried out in an open graphitecontainer, the required yhigh temperature being produced by a graphiteresistance heaterv which is spaced about the graphite container in whichthe fusion is carried out. A temperature of about 1750 C..is required tofuse quartz. This heater Anecessarily. is heated i to a highertemperature than the container or its contents. As aconsequence theouterlayer of the fusion reacted chemically with the graphite containerand-if the products y of the reactionv diffuse through the mass whichmay happen evenbeforethe quartz crystal pieces composing the chargebegin to coalesce, further reduction may set in, leaving a coatingaround the various pieces after they have fused together. The quality ofthe `material is deteriorated by this variation in composition and madeunfit for certain purposes, particularly for high grade optical devices.I have found that this condition may be substantially avoided by drawingoff products of reaction-very rapidl as they are formed and in a certainspecified' manner as described hereinafter. As aconsequence a high gradeproduct free from striae may be produced.

In carrying out my invention the quartz or silica is ooninedduring thefusing proc,- ess to a closed container, thus minimizing the effect ofcarbona-ceous vapors onthe silica fusion and also shielding theincandescent heater from the silica vapors'aris'ing from the fusion.'Even thou h the carbonaceous vapors may react with t e silica at thecontact surfaces this reaction is not allowed to penetrate the mass.

My invention may be better understood lby reference to the followingdescription taken in connection with the accompanying drawing whichshows in vertical section a furnace embodying myinvention.

Referring to the drawing, the charge 1 of silica (ordinarily consistingof fragments of quartz crystals) is contained in a cruible 2 which isplaced within a gas-tight receptacle 3. Both the Crucible and thereceptacle 3 may consist of suitable highly refractory material, such asgraphite or molybdenum or tungsten. Into the cover of the receptacle 3is inserted a graphitetube 4 through whichv gases may be exhausted.Surrounding there'ceptacle '3 is a tubular heater 5, ordinarilyconsisting of graphite. Electric contact is made at opposite ends of theheater 5 by` the terminals' 6 and 7 which may consist of copper. Acooling fluid may be supplied to these terminals by the pipes 8 and 9,part of the circulatory cooling system having been shown cut away toprovide a more simplified drawing. The container 2 for the quartz fusionrestson a graphite cup 10 filled with powdered graphite and acting asheat screen. Surrounding the exhaust tube` 4 and resting Vupon the upperend of the heater 5 `is a second graphite screen 11. A cylindricalgraphite screen 12' surrounds the heater 5.

This entire structure is supportedby the suspension rods ,13, V14, 15,16 from the cover17vof a steel casing 18, and therefore is maintainedsubstantially out of direct thermal contact with the outer enclosure.This outercasing has been shown in dotted lines as it may be omitted insome cases when vacuum fusion only is desired. The 'cover 17 is boltedto the tank 18 which is made vacuum-tight as described in my priorPatent No. 1,536,821 and as moreover well understood in vacuum furnacepractice. The/ water cooling tubes 8 and 9' are led through the cover17, the joints being made vacuum-tight as indicated.

The graphite exhaust pipe 4 is joined to a metal pipe 19 which isconnected in turn to an exhaust pump (not shown) by a valved pipe 24. Ifdesired the pipe 19 also may be connected to a source of gas underpressure by a valved pipe 20. Preliminary to making a uartz fusion thevalve21in the pi e 20 is closed and the vacuum valve22 to t e vacuumsystem is opened thereby withdrawing gas from the fusion chamber. VDueto the porosity of graphite, the gas in the space outside the housing 3in the tank 18 is exhausted at the same time by the gas seeping fromthis space through the interstices in the graphite and through thejoints between the graphite exhaust pipe and the receptacle 3. If it isdesired to obtain a slug fused only in vacuum or a slug made in vacuumand then subjected to slight pressure, I may do away with the steelcasing 18, and make the graphite vacuum system heavy walled, This systemmay then be suspended in the 4carbon tube furnace as well understood infurnace practice. Vacuum slugs such as are made in the usual way andlater extruded into the form of tube, rod, ribbon and the like from acarbon tube furnace, may be made in this manner, or the bubblesA in thisslug may be appreciably reduced for certain purposes by applying apressure of about 100 Vlbs. per sq. in. or even a greater pressure inthe graphite vacuum system. The required pressure may be obtained byclosing the vacuum valve 22 and admitting gas under pressure through thevalved pipe 20.

When the vacuum-fused product is to be subjected to higher pressures, asdescribed in my prior patent, the graphite walls of the.

container 3' and the pipe 4 may be made sulficiently thick-walled andtight atthe joints to withstand they required pressure. Plugs may beinserted in holes provided some- VWhere in this system as indicated at23.

When a gas under pressure is admitted, these plugs are blown out and thepressures inside and outside the graphite container are equalized. j

The following advantages are obtained by exhausting the immediate' spaceabout the quartz fusion by a conduit communicating with a closedreceptacle surrounding the fusion: First, the gases" evolved from thefusion"which attack the carbon at 'incandescence are removed at a morerapid rate ythan would be the case if they were permitted to diffusethroughout the space within the outer enclosing envelo e; secondly, thequartz fusion is protected rom the carbon vapors from the highlyincandescent heater, and in the third lace the heater is protected fromthe chemical effect of silica vapors and hence its life is greatlyprolonged.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is

1. An electric furnace comprising an enclosed charge-receivingreceptacle consisting ofgraphite, means for evacuating said receptacle,a vacuum-tight outer ,envelope Wholly enclosing said receptacle, and aheater external to said receptacle and contained within said outerenvelope.

2. An electric furnace comprising an enclosed charge-receivingreceptacle consisting of porous refractory material, a conduitconsisting of porous refractory material communicating therewith forevacuating said receptacle, means for electrically heating saidreceptacle, and a gasftight enclosing envelope, the porosity of the Wallof said receptacle and communicating conduit being sulicient to permitof the evacuation therethrough of the space in the outer envelope.

3. An electric,A furnace comprising a mechanically sealed metalreceptacle, a carbonaceous inner chamber which is closed but providesinterstices for the slow passage of t gas therethrough, a resistanceheater extertherein, a resistance heater surrounding said secondcontainer and means for exhausting gas directly from lsaidsecondcontainer, the porosity of said second container being sufcientlygreat to permit of the evacuation of ,the first container through saidsecond contalner. n

5.,.A. furnace comprising a sealed outer container, an inner container,a charge-receivmg receptacle therein, means for supporting said innercontainer spaced out of direct thermal contact with the outer container,a conduit provided with closures removable by internally appliedpressure communicating with said inner container and means for heatingsaid inner container.

6. An electric furnace comprising an outer metal container capable ofbeing made vacuum-tight, an inner container consisting of carbonaceousmaterial, means for suspending. said inner container from the wall ofthe outer container, a gas exhaust pipe extending from the exteriorthrough the Space between said containers and communieating with theinner container, a. radiant heater surrounding said carbonaceouscontainer, a heat screen interposed between said heater and the outercontainer.

In witness whereof, I have hereunto set my hand this ninth day of July,1925.

PHILIP K. DEVERS.

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